1. Field of the Invention
The invention relates to processes for cleaning and conditioning ceramic components of a semiconductor wafer processing system. More particularly, the invention relates to a method and apparatus for cleaning the surface of an electrostatic ceramic chuck and other ceramic components of semiconductor wafer processing equipment using a plasma.
2. Description of the Background Art
Plasma-based reactions have become increasingly important to the semiconductor industry, providing for precisely controlled thin-film depositions. For example, a plasma reactor in a high-temperature physical vapor deposition (PVD) semiconductor wafer processing system generally comprises a reaction chamber for containing a reactant gas, a pair of spaced-apart electrodes (cathode and anode) to generate an electric field within the chamber, and a substrate support for supporting a substrate within the electric field. The cathode is typically embedded within the substrate support, while the anode is embedded in a target material that is to be sputtered or deposited onto the substrate. The electric field ionizes the reactant gas to produce a plasma. The plasma, characterized by a visible glow, is a mixture of positive and negative reactant gas ions and electrons. Ions from the plasma bombard the target releasing deposition material. As such, a deposition layer forms on the substrate which is supported upon the surface of the substrate support just above the cathode.
A particular type of substrate support used in a high temperature PVD system is a ceramic electrostatic chuck. Ceramic electrostatic chucks create an electrostatic attractive force between the substrate (i.e., a semiconductor wafer) and the chuck to retain the wafer in a stationary position during processing. A voltage is applied to one or more electrodes imbedded within a ceramic chuck body so as to induce opposite polarity charges in the wafer and electrodes, respectively. The opposite charges pull the wafer flush against the chuck support surface, thereby electrostatically clamping the wafer. More specifically, in a "unipolar" electrostatic chuck, voltage is applied to an electrode embedded within the pedestal supporting the chuck. The voltage is referenced to some internal chamber ground reference. Electrostatic force is established between the wafer being clamped and the chuck. When the voltage is applied, the wafer is referred back to the same ground reference as the voltage source through a conductive connection to the wafer. Alternatively, the plasma generated in the chamber can reference the wafer to ground.
The ceramic material used to fabricate a high temperature chuck is typically aluminum-nitride or alumina doped with a metal oxide such as titanium oxide (TiO.sub.2) or some other ceramic material with similar resistive properties. This form of ceramic is partially conductive at high temperatures. Because of this characteristic, the wafer is primarily retained against the chuck by the Johnsen-Rahbek effect. Such a chuck is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992.
One disadvantage of using a chuck body fabricated from ceramic is that the characteristics of the chuck surface change over time. For example, exposing the chuck surface to organic material degrades chuck performance. Specifically, adventitious (surface) carbon, water and hydroxides collect on the chuck surface. Such contaminants enter the chamber during wafer processing as wafers are passed from a loadlock to the chamber or when the chamber is exposed to the atmosphere during a maintenance cycle. Additionally, outgassing of chamber components produces hydrocarbon contaminants e.g., O-rings inside the chamber breakdown and outgas. These contaminants produce a conductive carbon film on the chuck surface that grows if not removed. Additionally, waste products from wafer processing collect on the chuck surface causing contamination although these process waste products are not considered principle contaminants. The buildup of these contaminants reduces performance of the chuck and, after repeat processing and maintenance cycles, render the chuck useless (i.e., the chucking force is severely degraded and/or non-uniform). Premature replacement of the chuck results in higher unit cost and increased chamber downtime.
Therefore, a need exists in the art for a method of removing contaminant films that become deposited upon the support surface of the chuck as well as upon other ceramic components of a semiconductor wafer processing system.